An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the 802.11 IEEE Standards, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11g (2003), and the IEEE Standard 802.11n now in the process of being adopted, all of which are collectively incorporated herein fully by reference. These standards have been or are in the process of being commercialized with the promise of 54 Mbps or more effective bandwidth, making them a strong competitor to traditional wired Ethernet and the more common “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g or “802.11a/g” as well as the 802.11n standards achieve high data transmission rates using Orthogonal Frequency Division Modulation (OFDM) encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. OFDM is a digital multi-carrier modulation scheme that employs a large number of relatively closely spaced orthogonal sub-carriers. Each sub-carrier is itself modulated with a modulation scheme such as quadrature amplitude modulation, phase shift keying, etc., at a relatively low symbol rate. Even though data on a particular sub-carrier is modulated at a low symbol rate, the large number of sub-carriers provides an overall data rate similar to single-carrier modulation schemes that utilize the same bandwidth. An advantage of OFDM over single-carrier modulation schemes is its ability to cope with severe channel conditions such as, multipath and narrowband interference. For instance, the relatively low symbol rate allows the use of a guard interval between symbols to help manage time-domain spreading of the signal due to multipath propagation.
Generally, transmitters used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n standards as well as other standards such as the 802.16e IEEE Standard, perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers.
To better adjust to possible changes in environmental conditions and to varying channel quality requirements, a transmitter may adaptively change a modulation and coding scheme (MCS). To this end, the transmitter typically maintains a packet error count (PER) and selects an appropriate MCS based on the current PER value. More specifically, the transmitter may select an MCS corresponding to a higher data rate if the PER is relatively low or, conversely, selecting an MCS corresponding to a lower data rate if the PER is excessively high. However, MCS selection based on PER only yields a relatively low level of accuracy.